Fused silica capillary tubing has substantially replaced rigid, soft-glass capillaries as the separation column used in gas chromatography. This material offers higher strength, more flexibility, easier connect/disconnect capability, and a chemically inert inner surface for stationary phase application. It is typically used in 10 to 50 meter lengths wound in 15 to 20 cm diameter coils. Capillary bore diameters vary approximately from 0.01 to 0.6 mm with wall thicknesses in the range of 0.025 to 0.100 mm.
To better withstand handling and because the column is often heated in use in order to affect a better separation of unknown sample constituents, the silica capillary requires a strength protective coating that is able to withstand intermittent temperatures up to 400.degree. C. A polyimide material has been used for this purpose because it is one of the few classes of polymers able to withstand these temperatures.
The capillary is typically produced by heating the end of a relatively large diameter fused silica tube to its softening point and drawing the soft glass, thus reducing its size to capillary dimensions. The process is made continuous by slowly advancing the tube into the hot zone while simultaneously continuing to draw capillary out, the volume rate moving into the hot zone being equal to the volume rate being drawn out. The polyimide coating is applied in liquid form and cured to a polymerized solid in-line during the drawing process.
The combination of capillary tube diameter and coiled column diameter results in varying degrees of in-situ tensile stress in portions of the tube. Since the plymer coating is not impervious to water vapor, with time and repeated temperature cycling during use, the column ultimately fails mechanically as a result of the action of static fatigue on those capillary surfaces experiencing tensile stress. The present invention provides a simple capillary tube construction in which a stress balance is achieved that eliminates or reduces tensile stress on the capillary surfaces thus limiting the effect of static fatigue and resulting in a longer useful column life.